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Q : How much power does my telescope have?
A : A telescope has three types of power and they are measured against the performance of a normal human eye. They are magnifying power, light gathering power and resolving power. All three are important but the most important is resolving power. The longer the focal length of a telescope, the more a particular eyepiece will magnify the image. However, there is a practical magnification limit of 2x per mm of telescope aperture. Using an eyepiece which gives a magnification beyond that limit is normally of little use. The amount of light that a telescope can gather depends on size of the aperture and the more light that can be gathered, the better the resolution. What you will see through your telescope will then depend on these three powers. For example, compared to the human eye, and using the 2x per mm rule, a 150mm aperture telescope will have a maximum practical magnifying power of 300x, a light gathering power of 600x and a resolving power of 0.8 arc-seconds.
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